Stars' Twinkling Light Reveals Gravity Strength

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The way a star flickers can shed light on the strength of
gravity's pull on its surface, researchers say. And discovering
more about a star's pull can yield key insights on its
evolutionary state and on any planets that might orbit it,
scientists added.

Fluctuations in the light of sun-like stars are driven by many
factors, such as the presence of darker, cooler areas on its
surface. This spottiness or granulation results from the way the
material that makes up the stars rises and falls. The strength of
the
gravitational pull on the surface of that star can in turn
influence how great this churning gets.

The surface gravity of a star can in principle shed light on many
of its other properties, such as temperature and chemical makeup.
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"Once you know a star's surface gravity, then you only need one
other measurement, its temperature, which is pretty easy to
obtain, to determine its mass, size and other important physical
properties," study author Keivan Stassun, an astrophysicist at
Vanderbilt University in Nashville, said in a statement.

The surface gravity of most stars has often proven difficult to
measure accurately, with estimates potentially being as much as
150 percent off. Astronomers can deduce the surface gravity of
some bright stars with an uncertainty of just 2 percent by
analyzing rhythmic fluctuations in their light due to sound waves
zipping inside them, a strategy known as
asteroseismology. However, this method only works on several
hundred of the closest, brightest stars, a paltry fraction of all
the stars in the sky.

"Measuring stellar surface gravities well has always been a
difficult business," study author Gibor Basri at the University
of California, Berkeley, said in a statement. "So it is a very
pleasant surprise to find that the subtle flickering of a star's
light provides a relatively easy way to do it."

The researchers discovered they could deduce the surface gravity
of stars by looking at how their brightness varied by analyzing
high-precision measurements of more than 150,000 stars gathered
by NASA's
Kepler space telescope. They compared their results with the
surface gravity values of a few stars calculated independently
via asteroseismology

The scientists confirmed that granulation caused flickering in
stellar brightness on the scale of less than eight hours. This
granulation was in turn linked with surface gravity. The lower
the surface gravity, the more change in brightness was seen in
the flicker, probably because of the greater amount of churning
by the star's hotter, brighter matter and colder, darker
material.

"It turns out you can accurately measure a key fundamental
property of a star in a really straightforward and conceptually
simple manner," Stassun told SPACE.com.

This new technique apparently enables scientists to calculate the
surface gravity of sun-like stars with as little as 25 percent
uncertainty. Its major limitation is that it requires very high
quality data taken over long time periods, but this is exactly
the kind of information collected by Kepler while it was
searching for regular dips in starlight caused when alien planets
crossed in front of their stars.

This new technique could also help reveal clues about stellar
evolution. The researchers noted that as stars got older and
swelled to become red giants, their flickering became slower,
probably because their matter churned more slowly.

"The flickering of these older stars is slow and it's loud,"
Stassun said. "This is an interesting new way to look at stellar
evolution and a way to put our sun's future evolution into a
grander perspective."

This method could shed light on any worlds encircling these
distant stars. A common strategy for detecting and learning more
about such exoplanets is to
look at the effects of their gravitational pull on their stars.
Calculating the strength of the star's pull can therefore help
significantly improve estimates of the sizes of the hundreds of
exoplanets discovered in the last 20 years. Current estimates
have uncertainties ranging from 50 to 200 percent, and the
improved figures for the surface gravity of stars via this new
method should reduce these uncertainties by at least half,
researchers said.

"This actually could be the breakthrough we've needed to pin down
the sizes of hundreds more stars and exoplanets," Maria Womack,
the program director at the National Science Foundation, which
funded the research, said in a statement. "Getting accurate sizes
is critical to measuring exoplanet density, which has been a
missing puzzle piece for so many planets. So, in addition to
having implications for stellar evolution, this innovative work
will be invaluable for identifying hundreds of exoplanets as
either rocky or gaseous."

Stassun added, "We're now working to, using our method,
recalculate the surface gravities of
all the stars that Kepler has observed planets around. We want to
do a reassessment en masse of all the planets found by Kepler to
more accurately measure their properties."

"We have a whole new perspective now on sizing up stars, on how
they live out their lives, and all you have to do is watch the
stars twinkle," Stassun said.

The scientists detailed their findings in the Aug. 22 issue of
the journal Nature.